1. Field of the Invention
The present invention relates generally to control circuits and, more specifically, the present invention relates to control circuits used in power supplies that are responsive to a feedforward signal.
2. Background
Power supply control circuits may be used for a multitude of purposes and applications. Most power converter applications have both cost and performance goals. It is therefore necessary for control circuit features to be implemented that minimize the cost of external circuitry such as the input bulk capacitor in AC/DC converter power supplies. In addition the tolerances of the control circuit are important to provide consistent performance in the power supply application as well as further reduce power converter cost by reducing the design margins that need to be applied when the power converter is developed.
Power supplies typically comprise a power supply controller circuit, a switch coupled to an energy transfer element, a source of input voltage and one or more outputs. The power supply controller typically controls a switching of the switch to regulate energy delivered from the input to the output of the power converter in response to a feedback signal generated by a feedback circuit forming part of the power converter. Power supply controller circuits operating with pulse width modulator (PWM) modes of operation regulate the duty cycle of the switch as one technique to regulate energy delivered from the input to the output of the power supply. The duty cycle of the switch is the ratio of the switch on time to an overall switching cycle period defined by the power supply controller circuit.
Power supply controllers make use of feedforward signals in common power converter topologies such as forward converters and flyback converters. A feedforward signal is a signal whose magnitude is a function of the value of the input voltage to the power converter. In general therefore a feedforward signal can be regarded an input voltage signal that is representative of an input voltage to a power supply. A feedforward signal is typically used to provide a way to adjust the switch duty cycle independent of the feedback signal or in other words for a given or fixed feedback signal. In flyback converters, the advantage of a feedforward signal can for example be to reduce the size of a bulk capacitance at the input of an AC/DC power converter in particular for power supply controllers operating in the voltage mode of control where ripple voltage across the bulk capacitor is more difficult to filter. The ability to adjust the switch duty cycle independent of the feedback signal allows fast response to ripple voltage appearing across the bulk capacitor and therefore reduces ripple voltage appearing across the output of the power converter. Without the use of feedforward techniques, the power supply controller must respond to the feedback signal that responds to ripple voltage appearing across the power converter output and the power supply controller then controls the duty cycle of the switch accordingly to reduce the power converter output ripple voltage. This introduces delays and output ripple is therefore difficult to reduce without for example increasing the size of the input bulk capacitor. In forward converters a feedforward signal is typically used by the power supply controller to control the maximum duty cycle of the switch to ensure there is sufficient time to reset the flux in the magnetic core of the energy transfer element as will be known to one skilled in the art.